1. Field of the Invention
This invention relates to methods of and apparatus for the detection of inaccessible metallic objects, and, in particular, buried distribution lines for gas, water, electricity, telecommunications and the like.
2. The Prior Art
In order to facilitate the replacement and repair of buried distribution systems (lines) it is desirable to have a rapid and simple method of accurately locating their position. Most buried distribution systems are constructed either of ferrous or nonferrous metals, either alone or in combination with nonmetallic materials. The metal components of these systems frequently carry alternating currents which generate magnetic fields which can be detected. Alternatively, currents can be induced in these metals by an external transmitter and the currents detected to demonstrate the presence of the distribution lines.
One method for the detection of buried metallic objects is described in UK Pat. No. 1,299,183. A transmitter having an aerial consisting of a coil wound on a ferrite rod radiates a signal which is preferably at a frequency of between 100 Hz and 10 KHz. This signal energizes buried metallic objects which modify the pattern of the field. A receiver is tuned to the same frequency and has a similar ferrite rod aerial. This aerial, however, has its axis perpendicular to that of the transmitting aerial and is disposed so that it is not energized by the direct signal.
A second method, described in UK Pat. No. 1,509,380, has a number of coplanar pickup coils whose axes define the corners of a polygon on the ground. A receiver compares the phases of the signals induced in these coils and, from their relative phases, determines whether or not a cable or pipe is buried below that polygon. This receiver may be tuned to line power frequencies, since even gas and water pipes usually carry such currents resulting from stray earth return currents. Alternatively, an external transmitter may be used, and the receiver tuned to its radiated signal.
In UK Pat. No. 1,509,914 there is described yet another method for the detection of buried systems. In this method an array of detector coils scans the surface to detect the presence of a subterranean source of radiation by measuring the change in signal strength, a maximum indicating the position of the source.
One difficulty with scanning methods for the detection of buried radiators is that various metallic objects carry different currents and thus create fields of vastly different strengths. For example, an electricity line may carry a 50 Hz current ten thousand times greater than the stray earth return current in a water pipe. With an externally actuated system, the current induced in an insulated pipe may differ substantially from that induced in a bare pipe which is in contact with the soil. These different field strengths create problems with detector apparatus and it is often necessary to make several scanning passes, adjusting the gain before detecting the field maximum which indicates the position of the buried lines.
It is not possible to use a simple automatic gain control technique to overcome this variation in signal strength, since this would also mask the variations in signal from individual radiators which it is desired to detect. A multichannel detector system has, therefore, been devised to provide an instantaneous indication of the position of a maximum in an electromagnetic radiation field by comparison of the output levels of the different channels. Such a system is amenable to the application of automatic gain control which thus facilitates and speeds operation.